DE-A-42 12 604 discloses a process for conditioning organically polluted raw drinking water which is polluted with crop-protection agents, a UV treatment with light being performed. In this case, the technical problem underlying the disclosure is to provide a process, one of the purposes of which is to avoid sub-sequent oxidative treatment of the drinking water to be conditioned. In this case ultraviolet light having a wavelength of &gt;240 nm is used. If light which also has components below 240 nm is used, nitrite-destroying substances are simultaneously added.
The purpose of the process is to destroy organic compounds, in particular crop-protection agents such as atrazine, without converting nitrate to nitrite.
U.S. Pat. No. 5,122,496 discloses a device which treats water contaminated with nitrate and/or nitrite. In this case catalysts are used which have defined pore radii. A UV lamp is used to kill off microorganisms.
Owing to more stringent legal stipulations for drinking water quality, the problem arises for many waterworks of forcing residual nitrate quantities of 50 mg/l and above, as occur in sources presently used, below the legal guide values of 25 mg/l, and, what is more, at economically justifiable costs and without introducing substances which are not in any case usually present in normal drinking water.
To solve this problem, the present invention proposes a two-stage process: in a first step, the nitrate-containing water to be treated is irradiated in a thin layer with UV treatment light whose spectrum is chosen so that it is absorbed by nitrate. As a result, in the aqueous environment, nitrate is reduced to nitrite with simultaneous release of oxygen.
The wavelength of the UV light used in the first process step simultaneously corresponds to an absorption maximum of nitrate ions in an aqueous environment and also to an absorption minimum of water, the wavelength of the UV light being in the range between 200 and 230 nm and the pH in the first process step being in the range between 8 and 11.
In a second process step, the nitrite thus obtained is then completely reduced to nitrogen using a chemical reducing agent.
For the second, chemical step, use may be made of industrially manufactured inexpensive chemicals which are acceptable with regard to the use of drinking water, such as amidosulphuric acid. Obviously, no undesirable chemicals whatsoever are introduced into the water by the first step either.
In this manner, an effective and inexpensive reduction of the nitrate content is achieved overall.
Advantageous developments of the process according to the invention are specified in subclaims.
For the first, photochemical, process step, it is advantageous if the volume of water to be treated by the treatment light utilizes the quanta given off by the treatment light source as completely as possible. For this purpose, the invention proposes a device for carrying out the process which comprises the features listed in Claim 9.
Advantageous developments of this device are in turn the subject-matter of subclaims.